Field of the Invention
The invention relates to an offset printing method by which a printing form is produced from digital signals and in which, after the printing form is inked, a printed image formed of dot-matrix or screen pixels or half-tone dots is generated on a recording medium.
In conventional offset printing presses, inking can be affected by the variation in the thickness of the layer of ink produced on the printing surface of the recording medium. In offset printing presses with an ink duct roller, for example, the width of an ink strip on the ink duct roller can be varied and, as a result thereof, the volume of ink required in the offset printing press varies over the entire width of the printed image. For effecting a zonal or zonewise correction of the inking or ink feeding, inking devices with adjusting elements can be provided, which vary the thickness of the ink film or layer in zones transverse to the printing direction.
These variations in the thickness of the ink film or layer have a uniform effect over the entire length of the print. A local correction of the inking which would affect only certain regions of the printed image is not possible with such offset printing presses.
In offset printing presses wherein the instant of time at which lateral distribution may commence is variable, it is possible to achieve changes in coloration in the printing direction or circumferential direction (note published German Patent Documents DE 36 14 555 A1 and DE 40 04 056 A1). By varying the onset time of the lateral distribution, shifts in the extent of thickness of the ink layer occur over the entire width of the printed image. The application of such methods is limited to special cases, such as the printing of copies produced from one sheet, for example.
It has indeed become possible heretofore to exert an influence upon the inking or ink feeding in the production of the printed image when the printing form is being made. During the production of the printing form, methods known as "computer to plate" or "computer to press" take into account standardized printing characteristics, which describe the increase in tonal value in offset printing, and uniform ink layer thicknesses, in order to obtain half-tone images by means of dots of different sizes having a mutually equal spacing, or dots of equal size having a different spacing therebetween. By this method, printing forms are produced which are based on standardized parameters, i.e., parameters referred to a standard offset printing press. A conventional or usual offset printing press has characteristics, such as the increase in tonal value thereof, for example, which differ from these standardized parameters. The cause of these deviations may be the use of printing inks or rubber blankets other than those which are provided in the establishment of the standardized parameters, or the existence of temperature and humidity values which differ markedly from average values. Furthermore, the aforementioned ink corrections affect the ink layer thickness, which contradicts the assumption that the printing forms are produced with an ink layer or film of uniform thickness. The variation in the aforementioned parameters in the production of the printing form is not suitable for varying the inking or ink feeding dot by dot or regionally.
In generating printed images with the aid of a computer, it has become known heretofore to display the printed image with high resolution on a screen of a size which is as close as possible to the format to be printed. The generated images can be varied arbitrarily interactively with image processing or imaging programs and can be visually inspected by so-called soft proofing; a desired version of the images can then be stored in memory in the form of digital image data. The digital image data can be used directly in applying images to a printing form carrier, for which purpose the device for generating the printed image can be connected directly to the open or closed-loop control device, which regulates with human intervention or controls fully automatically, of a printing press, at least one image-applying or imaging unit being connected to the open or closed-loop control device.
Despite the possibility of varying the inking or ink feeding dotwise or regionally in the soft proof on the screen, the deviations in the characteristics of a particular offset printing press from those of the aforementioned standard offset printing press are not taken into account herein because, at the instant of time the soft proof is produced, no signals or data regarding such deviations are available.
U.S. Pat. No. 5,163,368 shows an offset printing press which operates in accordance with the "computer to press" principle. When individual pixels are generated on the printing form carrier due to removal by laser beam or electro-erosion, the cyclical mechanical errors of the printing device itself are taken into account. In this regard, test print-outs are produced with a test plate which includes a test pattern, and deviations in the position of pixels from the theoretical ideal position are ascertained on these test print-outs. These coordinate-dependent deviations are stored in memory and are taken into account in the correction of the coordinates of the pixels which are to be generated on a printing form carrier. The image applying or imaging unit is thus calibrated with respect to the coordination of the pixels before the imaging is performed.
In the foregoing method, no errors based on cyclical mechanical errors of the offset printing press are present in the printed image. More extensive corrections in inking or ink feeding which involve specific regions in the printed image and take into account the aforementioned print-specific characteristics are not considered in the foregoing reference.
In offset printing, it is a conventional practice for a customer to evaluate the print quality after test copies have been printed on the offset printing press, and then to approve the production or print run. Inking corrections can be made by adjusting the elements which affect the inking or ink feed. However, this is costly both in time and material, because such ink corrections cannot be evaluated until after about 100 copies have been printed and, as a rule, after several correction steps have been performed. Moreover, dotwise correction cannot be performed. As a substitute therefor, test copies or proofs can be made relatively quickly and inexpensively for evaluating the printing results. Disadvantageously, however, they are produced with different types of printing equipment, so that the proofs differ in color from copies made in the offset printing press, and thus mistakes in the evaluation of the print quality result.